People are generally aware of the most common causes of hallucinations, like schizophrenia and a really high fever. But lesser known are the more rare causes; here are six you probably haven't heard about.

Mentally Ill Patients Abuse Alcohol, Tobacco, And Drugs More Than The General Population

Research in the past has linked psychiatric disorders to certain high-risk behaviors. However, the prevalence of alcohol, drug, and tobacco abuse among people with a mental illness seldom receives attention. A study out of Washington University School of Medicine in St. Louis and the University of Southern California determined that substance use is higher among psychiatric patients compared to people in the general population.

"With public health efforts, we've effectively cut smoking rates in half in healthy people, but in the severely mentally ill, we haven't made a dent at all," first author Sarah M. Hartz, M.D., Ph.D., assistant professor of psychiatry at Washington University, wrote in the study, published in JAMA Psychiatry. "I take care of a lot of patients with severe mental illness, many of whom are sick enough that they are on disability. And it's always surprising when I encounter a patient who doesn't smoke or hasn't used drugs or had alcohol problems."

Dr. Hartz and her colleagues from Washington University and USC recruited 20,000 participants for the study including 9,142 people with a psychiatric disorder and over 10,000 healthy individuals. Psychiatric patients had been diagnosed with schizophrenia, bipolar disorder, or schizoaffective disorder, a condition characterized by hallucinations, delusions, and mood disorders. The research team gauged how often each participant drank heavily, smoked marijuana heavily, used nicotine, or used other recreational drugs.

Around 30 percent of psychiatric patients engaged in binge drinking as oppose to eight percent of the general population. Seventy-five percent of the participants diagnosed with a mental illness were also considered regular smokers, while 33 percent of healthy participants admitted to smoking regularly. Another 50 percent of psychiatric patients admitted to using marijuana regularly while half of the group admitted to using other illicit drugs. Eighteen percent of the general population said they smoked marijuana on a regular basis, and 12 percent owned up to illicit drug use.

"These patients tend to pass away much younger, with estimates ranging from 12 to 25 years earlier than individuals in the general population," Dr. Hartz said. "They don't die from drug overdoses or commit suicide — the kinds of things you might suspect in severe psychiatric illness. They die from heart disease and cancer, problems caused by chronic alcohol and tobacco use."

Researchers were also shocked to learn that protective factors, such as race and gender, had no effect on their ability to avoid substance abuse if they developed a psychiatric disorder. For example, Hispanics and Asians have a better chance of avoiding alcohol, tobacco, or drug abuse compared to European Americans and women, who are less likely to develop a substance use disorder compared to men. "We see protective effects in these subpopulations," Hartz explained. "But once a person has a severe mental illness, that seems to trump everything."

According to the National Alliance on Mental Illness, health care professionals estimate that 50 percent of the mentally ill population also suffers from substance abuse. Incidences of drug dependency among psychiatric patients are higher among men and those between the ages of 18 and 44. Health care professionals specializing in mental illnesses, and the family of psychiatric patients are unaware of just how common substance abuse is among people with a psychiatric disorder. Dr. Hartz and her team hopes more attention will be given to helping psychiatric patients steer clear of alcohol, tobacco, or drugs.

"Some studies have shown that although we psychiatrists know that smoking, drinking, and substance use are major problems among the mentally ill, we often don't ask our patients about those things," Dr. Hartz added. "We can do better, but we also need to develop new strategies because many interventions to reduce smoking, drinking, and drug use that have worked in other patient populations don't seem to be very effective in these psychiatric patients."